1. Field of the Invention
The field of the invention is that of bipolar transistors and especially vertical structure heterojunction bipolar transistors in which the current is conveyed perpendicularly to the surface of the substrate, these transistors being used for microwave applications.
They are formed by a stack of layers of semiconductor materials, at least one surface layer of which is etched in "mesa" form, namely in a raised or embossed pattern. FIG. 1 describes the conventional structure of a heterojunction bipolar transistor or HBT whose emitter, base and collector are at three different levels. Conventionally, a contact C.sub.E is made on the surface of the emitter by the superimposing of matched layers. Laterally, two contacts C.sub.B1 and C.sub.B2 are made in the base on either side of the emitter and two contacts C.sub.C1 and C.sub.C2 are also made in the collector on either side of the base.
This type of vertical structure raises a problem of electron/hole recombinations at the free surfaces s.sub.1 and s.sub.2 shown in FIG. 1. These recombinations play a major role in the deterioration of gain in current.
The smaller the transistors, the greater is this phenomenon (through leakage of electrons at the edges). The microwave applications of vertical structures of this kind are therefore highly penalized by this problem. Indeed, for operation in microwave applications, a bipolar transistor consists of several parallel-connected elementary transistors (also called fingers) and in order to limit the base resistance of an elementary transistor, it is necessary to limit the width of the emitter. Typically, the width of a finger must be in the range of 2 .mu.m for applications at frequencies below 100 GHz while the length of the emitter finger may be about 30 microns.
2. Description of the Prior Art
There have already been certain solutions brought to bear on this problem, especially for HBT transistors made with GaAlAs/GaAs materials. In particular, one approach envisaged has been that of introducing a passivation layer between the emitter and the base creating an intermediate depopulated zone in which the electron/hole recombinations are made practically impossible because of the disappearance of the free carriers in this zone. More specifically, FIG. 2 shows a structure in which the emitter has a particular architecture obtained by partial etching of a GaAlAs layer deposited on a p doped GaAs layer constituting the base. Two contacts C.sub.B1 and C.sub.B2 are made at the emitter level on the surfaces s'.sub.1 and s'.sub.2. Then, by an appropriate processing operation, these contacts may be diffused over the thickness e.sub.o. In this configuration, the electron/hole recombinations in the base, which were earlier fostered on the free surface of the base, are limited because of the emitter thickness e.sub.o kept above the base. The problem with this type of structure however lies in its poor stability inasmuch as the diffusion made for the contacts cannot be entirely controlled and may therefore continue to develop with time and temperature.
This is why the invention proposes an optimized heterojunction bipolar transistor structure in which the surface recombinations are minimized through a particular emitter architecture.